Polarization-based processing of unpolarized image light

ABSTRACT

A display system includes a source of unpolarized image light, and a stack of polarization-selective optical elements operable to switchably convert the unpolarized image light into two orthogonally polarized light beams, each of which being switchable in at least one beam characteristic. An output polarizer selects one of the two orthogonally polarized light beams for providing to a user. A depolarizer may be disposed between an electronic display emitting polarized light and the stack. The depolarizer may be in the form of an LC bilayer with randomized in-plane optic axis.

REFERENCE TO RELATED APPLICATION

The present application claims priority from U.S. Provisional PatentApplication No. 62/899,431 filed on Sep. 12, 2019, entitled “Depolarizerfor Near Eye Display”, and incorporated herein by reference in itsentirety.

TECHNICAL FIELD

The present disclosure relates to display devices, and in particular tonear-eye displays and components thereof.

BACKGROUND

Head mounted displays (HMD), helmet mounted displays, near-eye displays(NED), and the like are being used increasingly for displaying virtualreality (VR) content, augmented reality (AR) content, mixed reality (MR)content, and they are finding applications in diverse fields includingentertainment, education, training and biomedical science, to name justa few examples. The VR/AR/MR content can be three-dimensional (3D) toenhance the experience and to match virtual objects to real objectsobserved by the user. Eye position and gaze direction, and/ororientation of the user may be tracked in real time, and the displayedimagery may be dynamically adjusted depending on the user's headorientation and gaze direction, to provide a better experience ofimmersion into a simulated or augmented environment. One or morevarifocal lenses may be used to dynamically adjust the focused imagelocation for each eye, e.g. to reduce a discrepancy between eye vergenceand visual distance accommodation known as vergence-accommodationconflict.

Compact display devices are desired for head-mounted displays. Because adisplay of HMD or NED is usually worn on the head of a user, a large,bulky, unbalanced, and/or heavy display device would be cumbersome andmay be uncomfortable for the user to wear. In order to reduce the sizeand/or weight of the HMD or NED, polarization-based optics using thinliquid crystal (LC) layers and stacks may be used to implement lensesand other light processing devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings, in which like elements are indicated with like referencenumerals, which are not to scale, and in which:

FIG. 1 is a schematic perspective view of a liquid crystal (LC)polarization device;

FIG. 2 is a schematic cross-sectional view of an electrically controlledactive LC device;

FIG. 3A is a schematic diagram illustrating example liquid crystalorientations in the plane of the LC layer of an LC PBP lens;

FIG. 3B is a schematic diagram illustrating example liquid crystalorientations in the plane of the LC layer of an LC PBP grating;

FIG. 4A is a schematic diagram illustrating the operation of an examplePBP lens for RHCP light;

FIG. 4B is a schematic diagram illustrating the operation of an examplePBP lens for LHCP lightly;

FIG. 5 is a schematic diagram illustrating the operation of a PBPgrating for RHCP and LHCP light;

FIG. 6 is a schematic side cross-sectional view of a virtual reality(VR) HMD with an optical block focusing image light for the user;

FIG. 7 is a schematic diagram illustrating the operation of a PBP lensin cooperation with a switchable HWP;

FIG. 8 is a schematic diagram illustrating the operation of a varifocallens formed as a switchable PBP stack with the first PBP elementreceiving polarized light;

FIG. 9 is a schematic diagram illustrating an integrated implementationof the switchable PBP stack of FIG. 8;

FIG. 10 is a schematic diagram illustrating ring artifacts that mayappear in a display using the switchable PBP stack of FIG. 8 operatingon polarized image light;

FIG. 11 is a schematic cross-sectional view of a switchable PBP stackfor operating with unpolarized incident light;

FIG. 12A is a schematic diagram of a switchable PBP stack with activePBP elements of a same handedness separated by a passive HWP;

FIG. 12B is a schematic diagram of a switchable PBP stack with activePBP elements of opposite handedness stacked in a direct sequence;

FIG. 13 is a schematic side cross-sectional view of an HMD using theswitchable PBP lens stack of FIG. 11 coupled to an electronic displayemitting unpolarized image light;

FIG. 14 is a schematic side cross-sectional view of an HMD having adepolarizer disposed between a switchable PBP lens stack and anelectronic display emitting polarized image light;

FIG. 15 is a schematic side cross-sectional view of an HMD having adepolarizer disposed downstream of a pancake lens and upstream of aswitchable PBP lens stack;

FIG. 16 is a schematic cross-sectional view of a switchable PBP lensstack including an input depolarizer;

FIG. 17 is a schematic diagram illustrating a waveplate with spatiallyvarying in-plane optic axis orientation for operating as a depolarizer;

FIG. 18 is a schematic diagram illustrating an LC alignment layer withspatially varying alignment direction for use in an LC depolarizer;

FIG. 19 is a schematic cross-sectional view of a depolarizer formed withan LC bilayer having antisymmetric twist and spatially randomized opticaxis in the plane of the waveplate;

FIG. 20A is a schematic plan view of one depolarizer segmentillustrating the LC director twist in a lower layer of the LC bilayer;the LC director at the top of the layer is shown with a solid arrow;

FIG. 20B is a schematic plan view of the depolarizer segment of FIG. 20Aillustrating the LC director twist in the second layer of the LCbilayer; the dotted arrow illustrates the LC director at the bottom ofthe layer at the boundary with the lower layer;

FIG. 21A is an isometric view of a head-mounted display of the presentdisclosure; and

FIG. 21B is a block diagram of a virtual reality system including theheadset of FIG. 21A.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art. All statements herein reciting principles,aspects, and embodiments of this disclosure, as well as specificexamples thereof, are intended to encompass both structural andfunctional equivalents thereof. Additionally, it is intended that suchequivalents include both currently known equivalents as well asequivalents developed in the future, i.e., any elements developed thatperform the same function, regardless of structure.

As used herein, the terms “first”, “second”, and so forth are notintended to imply sequential ordering, but rather are intended todistinguish one element from another, unless explicitly stated.Similarly, sequential ordering of method steps does not imply asequential order of their execution, unless explicitly stated. The terms“NED” and “HMD” may be used herein interchangeably to refer tohead-wearable display devices capable of providing VR/AR/MR content tothe user.

An aspect of the present disclosure relates to an optical depolarizerand its use in a display system. In some implementations the displaysystem may be configured to be mounted upon a user's head for near-eyedisplay of images.

An aspect of the present disclosure relates to a display systemcomprising a stack of polarization-selective optical elements, the stackcomprising one or more switchable polarization-selective elements, thestack configured to operate on unpolarized image light in the absence ofinput polarizer to output two orthogonally polarized light beams havingswitchable beam characteristics. The stack may be followed by a clean-uppolarizer for selecting one of the two orthogonally polarized beams witha desired beam characteristic for forming an image.

In some implementations the stack comprises a sequence of liquid crystal(LC) Pancharatnam-Berry phase (PBP) optical elements, each paired with aswitchable LC HWP. In some implementations at least one of thepolarization-selective elements of the stack comprises a passive or aswitchable LC PBP optical element. In some implementations the stackcomprises multiple layers having a polarization-selective opticalproperty, wherein orthogonal polarizations are differently processed andboth passed to a next layer. In some implementations the stack comprisesmultiple LC layers. In some implementations the stack comprises asequence of LC PBP lenses of differing nominal optical power. In someimplementations the stack comprises one or more LC PBP lenses and one ormore active LC waveplates, wherein at least one of the LC PBP lenses isdisposed to receive unpolarized light. In some implementations the stackcomprises one or more LC PBP gratings and one or more active LCwaveplates, wherein at least one of the LC PBP gratings is disposed toreceive unpolarized light. In some implementations the stack comprises,or is followed by, a clean-up polarizer.

An aspect of the present disclosure relates to a display systemcomprising: a source of unpolarized image light; a stack ofpolarization-selective optical elements disposed to receive theunpolarized image light and operable to switchably convert theunpolarized image light into two orthogonally polarized light beams;and, an output polarizer disposed to receive the two orthogonallypolarized light beams and configured to select one of the twoorthogonally polarized light beams for forming an image. Each of the twoorthogonally polarized light beams may be switchable in at least onebeam characteristic. In some implementations the at least one beamcharacteristic may comprise a beam convergence characteristic. In someimplementations the at least one beam characteristic may comprise a beamdeflection angle. The display system may be configured to be mountedupon a user's head for near-eye display of images.

In some implementations the stack may comprise one or morePancharatnam-Berry phase (PBP) optical elements, wherein at least one ofthe one or more PBP optical elements is disposed to receive theunpolarized image beam. In some implementations one or morePancharatnam-Berry phase (PBP) optical elements may be switchable. Insome implementations one or more PBP elements may each be directlyfollowed by a switchable HWP. In some implementations the one or morePBP optical elements may comprise a plurality of PBP lenses of differingnominal optical power, and the stack may be configured to operate as avary-focal lens. In some implementations the plurality of PBP lenses maycomprise a liquid crystal (LC) PBP lens switchable to a neutral statelacking optical power. In some implementations the stack may comprise aplurality of switchable half-wave plates (HWP). In some implementationsthe PBP lenses may be sequentially paired with the switchable HWPs; insome implementations each of the switchable HWPs may directly follow oneof the PBP lenses. In some implementations the one or more PBP opticalelements may comprise a polarization grating. In some implementationsthe polarization grating may be directly followed by a switchable HWP.In some implementations the polarization grating may comprise an LC PBPgrating switchable to a non-diffracting state.

In some implementations the source of unpolarized image light comprisesan electronic display configured to emit unpolarized light. In someimplementations the source of unpolarized image light comprises anelectronic display configured to emit polarized image light, and adepolarizer disposed between the electronic display and the stack. Insome implementations the polarized image light is linearly polarized,and the depolarizer comprises a half-wave plate (HWP) with a randomizedin-plane optic axis. In some implementations the depolarizer may bedisposed adjacent to the electronic display.

In some implementations of the display system the depolarizer comprisesa birefringent layer with an in-plane optic axis which orientationvaries randomly or pseudo-randomly from one location to another in theplane of the birefringent layer, a property that may be referred to as arandomized in-plane optic axis. In some implementations the birefringentlayer has a retardance that is substantially constant across thewaveplate. In some implementations the birefringent layer has ahalf-wave optical retardance. In some implementations the orientation ofthe in-plane optic axis varies randomly or pseudo-randomly along thebirefringent layer. In some implementations the birefringent layercomprises LC material. In some implementations the birefringent layercomprises twisted nematic LC material. In some implementations thebirefringent layer comprises a plurality of homogenous area segmentseach having an optic axis, wherein the orientation of the optic axischanges randomly or pseudo-randomly between adjacent area segments. Insome implementations the depolarizer comprises a bilayer of twisted LCmaterial, the bilayer comprising two layers of opposite chirality. Insome implementation each of the two layers of the bilayer has ahalf-wave retardance.

An aspect of the present disclosure provides a method forpolarization-based processing of image light in a display system, themethod comprising: sequentially passing unpolarized image light througha plurality of polarization-selective optical elements to obtain twoorthogonally polarized light beams, each of which being switchable in atleast one beam characteristic; and, using an optical polarizer disposeddownstream of the plurality of polarization-selective optical elementsto select one of the orthogonally polarized light beams as an outputlight beam. In some implementations the method may comprise passingpolarized image light from an electronic display through a depolarizerto obtain the unpolarized image light for providing to the sequencepolarization-selective optical elements. In some implementations thesequentially passing unpolarized image light may comprise passing theunpolarized image light through a sequence of PBP lenses of differentnominal optical powers. In some implementations the sequentially passingunpolarized image light may comprise passing the unpolarized image lightthrough a sequence of LC polarization gratings.

An aspect of the present disclosure relates to a method of fabricating adepolarizer comprising exposing an alignment layer that comprises aphotosensitive material to polarized UV light, so that adjacent areas ofthe alignment layer are exposed to the polarized UV light of differentpolarization direction. In some implementations the method may compriseexposing a plurality of area segments of the alignment layer to thepolarized UV light so that the polarization of the UV light changesrandomly or pseudo-randomly when exposing adjacent area segments. An LCmaterial may then be deposited onto the exposed alignment layer to atarget thickness to obtained an LC layer having a randomized LC directororientation. In some implementations the target thickness corresponds toa half-wave retardance. The method may include polymerizing the LCmaterial after the depositing to obtain an LC layer with a fixedrandomized LC director orientation.

An aspect of the present disclosure relates to a depolarizer comprisinga birefringent layer with a spatially varying orientation of an in-planeoptic axis. In some implementations the orientation of the in-planeoptic axis varies randomly or pseudo-randomly along the birefringentlayer. In some implementations the birefringent layer has a retardancethat is substantially constant across the waveplate. In someimplementations the birefringent layer has a half-wave opticalretardance. In some implementations the birefringent layer comprises LCmaterial. In some implementations the birefringent layer comprisestwisted nematic LC material. In some implementations the birefringentlayer comprises a plurality of homogenous area segments, wherein theorientation of the optic axis changes randomly or pseudo-randomlybetween adjacent area segments. In some implementations the birefringentlayer of the depolarizer is a bilayer comprised of two layers of twistedLC material of opposite chirality.

An aspect of the present disclosure provides a depolarizer comprising: asubstrate; a bilayer of a twisted LC material disposed over thesubstrate, the bilayer comprising: a first layer of the twisted LCmaterial disposed over the substrate, the first layer having an LCdirector with a first sense of twist in a thickness direction, whereinthe thickness direction is normal to the substrate; and a second layerof the twisted LC material disposed over the first layer, the secondlayer having an LC with a second sense of twist in the thicknessdirection, wherein the second sense of twist is opposite to the firstsense of twist. The LC director of the first layer at an interface withthe second layer has a direction that varies randomly or pseudo-randomlyin a plane of the substrate. The bilayer may have a half-wave retardancethat is generally constant in the plane of the substrate. In someimplementations, the first layer and the second layer of the bilayer mayhave LC twist parameters optimized for broadband operation.

With reference to FIGS. 1-4, embodiments described herein may utilizeliquid crystal (LC) based devices that operate in apolarization-sensitive manner without substantially discriminatingbetween orthogonal polarizations in transmitted optical power. Suchdevices include, but are not limited to, LC PBP lenses, LC PBP gratings,and LC polarization switches. Referring first to FIG. 1, LC devicesdescribed herein may be in the form of, or include, an LC layer 31,which may be supported by a transparent or reflective substrate 30.Substrate 30 may be flat or curved. The polarization properties of suchdevices may depend on the material properties of the LC layer 31,orientation of LC molecules 35 within the layer, the chirality of the LCmolecules 35, the thickness of the LC layer 31, and the wavelength λ ofincident light. A predominant orientation of the LC molecules at anylocation (x,y,z) in the LC layer may be conveniently represented by aunit vector n(x,y,z) termed an LC director, n(x,y,z)=−n(x,y,z). Here aCartesian coordinate system (x,y,z) is used for convenience in which the(x,y) plane is parallel to the plane of the LC layer 31. Within the LClayer 31 the orientation of the LC director may be defined at least inpart by an alignment layer or layers 37 that may be disposed immediatelyadjacent to the LC layer 31. An LC device in which the orientation of LCmolecules is generally uniform across the LC layer may operate as awaveplate retarder. For incident light of a specific polarization, an LCdevice in which the orientation of the LC director varies in the planeof the layer may function, non-exclusively, as a lens, as a grating, oras a de-polarizer as described below, depending on the LC directororientation pattern.

An LC device may be active, where the LC material orientation iselectrically controlled, or passive, where the LC material orientationis fixed in place via material properties, for example by the alignmentlayers and/or by a polymer mixed into the LC fluid and cured at aparticular orientation within the LC layer.

Referring to FIG. 2, an active LC device may be constructed with the LClayer 31 sandwiched between two electrodes 39, at least one of which istransparent in the wavelength range of intended operation. Inembodiments operating in transmission, both electrodes 39 may beoptically transparent. Transparent electrodes 39 may for example be inthe form, or include, ITO (indium tin oxide) layers. In the absence ofvoltage between the electrodes, the LC molecules 35 may be oriented in adefault pattern that imposes desired birefringence properties on thedevice, for example a desired uniform or non-uniform retardance.Applying a sufficient voltage V between the electrodes 39 may reversiblyre-align LC molecules 35 in a way that changes birefringent propertiesof the LC layer. For example, in some LC materials applying a sufficientvoltage V to the electrodes 39 may align the LC molecules along theelectric field, as indicated at 35 a in the figure, so that the LC layer31 will lose its birefringence for light at normal or close to normalincidence. An example of an active LC device is an active waveplatewhich retardance may be switched off and back on by applying a voltage Vand by turning the voltage off, respectively. For example, an active LCdevice may be constructed to provide a retardance of a half-wave plate(HWP) in the absence of applied voltage, and substantially zeroretardance when a sufficient voltage V is applied. One or moreembodiments described herein may utilize such switchable HWPs,hereinafter referred to as s-HWP, as a polarization switch for polarizedlight. For example a suitably oriented s-HWP may reverse the chiralityof circular polarized (CP) light incident thereon in the absence ofvoltage (OFF state), and may leave the incident polarization stateunchanged in the presence of voltage (ON state). The relationshipbetween the applied voltage and the polarization action of an LCwaveplate may be reversed in other embodiments.

Referring to FIG. 3A, a Pancharatnam-Berry phase (PBP) lens 40 withdesired polarization processing and focusing properties may befabricated with the orientation of LC molecules 45 radially varying inplane of the LC layer and, possibly also in in the direction normalthereto, i.e. relative to an optical axis of the LC device (z-axis inFIGS. 1-4). The LC azimuth angle θ, i.e. the angle of rotation of aprojection of the LC director onto the plane (x,y) of the LC layer, mayvary radially from a center 41 to an edge 44 of the lens 40, with avaried pitch 43 Λ. The pitch Λ indicates a distance across which theazimuth angle θ of the LC director is rotated by 180°, and may be afunction of the radial distance r from the center 41 of the PBP lens.Polarized light beam passing through such lens experiencesradius-dependent retardation that adds a varying phase shift across thebeam's wavefront, resulting in a lensing action for a suitably selectedprofile of the LC orientation. In some embodiments the azimuth angle θof the LC orientation in the PBP LC lens 40 may vary in accordance withthe equation

${\theta (r)} = \frac{\pi \; r^{2}}{2f_{0}\lambda_{0}}$

where f₀ corresponds to the focal length of the PBP lens 40, and λ₀corresponds to the wavelength of incident light on the lens. In otherembodiments the tilt angle ϕ of the LC molecules of an PBP lens, i.e.the angle describing the molecules' tilt relative to the optical axis ofthe lens, may be radially varying to provide a desired phase profile.Such a lens may be either active, where the LC material orientation iselectrically controlled, or passive, where the LC material orientationis fixed in place via material properties and/or alignment layers. Anactive LC PBP lens may be constructed as described hereinabove withreference to FIG. 2. For example, optical power and polarizationswitching property of an active LC PBP lens may be turned off byapplying a suitable voltage across the LC layer to switchably align theLC molecules along the optical axis of the lens (z-axis). The state ofan active LC PBP lens in which it has a substantially zero opticalpower, or am optical power that is smaller than a threshold value, maybe referred to as a neutral state. The state of an active LC PBP lens inwhich it has a desired non-zero nominal optical power may be referred toas a neutral state.

Referring to FIG. 3B, an LC device 60 in which the orientation of the LCdirector varies periodically or nearly periodically in one dimension(1D) along a specific direction in the plane of the LC layer mayfunction as a polarization grating. A polarization grating may directincident light at an angle that depends on the grating's pitch and apolarization state of the incident light. One example of an LCpolarization grating is a PBP grating, in which grating ‘groves’ areformed by spatially varying birefringence in the plane of the grating.The LC director, which in the figure is represented by “LC molecules”65, in such grating varies its orientation in the plane of the LC layer,indicated in the figure as an (x,y) plane, defining a devicebirefringence profile in the plane of the LC layer. The azimuth angle θof the LC director 65 in the plane of the grating continuously changesfrom one edge to the other, typically with a fixed pitch 63. An LC PBPgrating may be either active, where the LC material orientation iselectrically controlled, or passive, where the LC material orientationis fixed in place via material properties and/or alignment layers orpassive. An active LC PBP grating may be constructed as described abovewith reference to FIG. 2, so that its diffractive power may be switchedOFF.

FIGS. 4A and 4B illustrate the operation of an example PBP lens 50, witha focal length f, for left-handed circular polarized (LHCP) light (FIG.4A) and right-handed circular polarized (RHCP) light (FIG. 4B). In thisexample, PBP lens 50 has a positive optical power for LHCP light whileswitching its polarization to RHCP, and a negative optical power forRHCP light while switching its polarization to LHCP. Thus a collimatedLHCP beam 51 exits the lens as a converging RHCP beam that converges toa focus at a distance f from the lens, while a collimated LHCP beam 53exits the lens as a divergent LHCP beam that appears to diverge from avirtual focus at a distance −f from the lens. The focal length f of thePBP lens defines its nominal optical power 1/f.

Referring to FIG. 5, a PBP grating 70 may be configured to deflect RHCPlight by a diffraction angle θ_(d) in one direction, and to deflect LHCPlight in an opposite direction, generally by the same diffraction angleθ_(d). In both cases the PBP grating 70 switches the circularpolarization to its orthogonal polarization. The pitch of an LC PBPgrating may be configured to provide a desired magnitude of thediffraction angle θ_(d). Such a grating may be either active, where theLC material orientation is electrically controlled, or passive, wherethe LC material orientation is fixed in place via material propertiesand/or alignment layers. An active LC PBP grating may be constructed asdescribed hereinabove with reference to FIG. 2. For example, an activeLC PBP grating may deflect incident CP light by the diffractionangle+\−θ_(d) depending on the chirality of incident light whilesimultaneously reversing its chirality in the absence of voltage (OFFstate), and may leave both the direction of propagation and thepolarization state of incident light unchanged in the presence ofvoltage (ON state). A PBP grating is an example of a polarizationgrating. Another example of a polarization grating is a volumeholographic LC grating, in which the orientation of the LC layermaterial may vary both in the plane of the LC layer and in the directionnormal to the LC layer, which may be referred to as the thicknessdirection. Such gratings may be constructed to selectively deflect onlyone of two orthogonal linear polarizations, without substantiallychanging the propagation direction of the other of the two orthogonalpolarizations. The volume holographic LC grating may operate, forexample, as an active element where the LC material is electricallycontrolled, and/or as a passive element, together with a linearpolarizer and an active polarization rotator operable to switch thepolarization status of the incident light. Embodiments described belowwith reference to LC PBP gratings may be modified to use such volumeholographic LC gratings instead.

Referring now to FIG. 6, there is schematically illustrated, in apartial cross-section, an example HMD 100 in which stacks of activeand/or passive LC devices such as those described above can be used. HMD100 includes an electronic display 153 disposed at a frontal side 152 ofa rigid body 151, facing an eyebox 157. The eyebox 157 defines theposition of an eye 160 of the user when the user wears HMD 100. Anoptics block 155, which is disposed in an optical path between theelectronic display 153 and the eyebox 157, transmits image light fromthe electronic display 153 to the eyebox 157. In one or more embodimentsthe optics block 155 may form a magnified virtual image of the pixelatedlight-emitting face 154 of the electronic display 153, typically fartheraway from the eyebox 157 than the electronic display 153. The virtualimage of the light-emitting face 154 of the display is then projected bya lens 161 of the eye 160 onto a retina 163 to form an image thereon.Although only a single optics block 155 is shown, the HMD 100 mayinclude two instances of this block, one for each eye of the user, andmay also include two instances of the electronic display 153. Theelectronic display 153 may be a pixelated display, for example, amicro-display with a total pixel count that may be smaller than, forexample, a pixel count of a conventional direct-view LED TV display. HMD100 may also include various other elements, such as one or morepositions sensors, one or more locators, an inertial measurement unit(IMU), and so forth, which may be coupled to the rigid body 151, and insome instances may be at least in part positioned at the frontal side152 thereof. In one or more embodiments HMD 100 may include one or morecameras 159, which may be configured for eye tracking and/or displaycalibration, and may be disposed downstream of the optics block 155. Thecamera(s) 159 may also be disposed upstream the optics block, or beintegrated into the optics block 155. The electronic display 153 may be,for example, an LCD display, an OLED display, an AMOLED display, or anyother suitable display. In some embodiments the electronic display 153may be configured to emit polarized light. In other embodiments theelectronic display 153 may be configured to emit unpolarized light. Theelectronic display 153 may be operationally coupled to a displayprocessor 170. In operation, the electronic display 153 receives imageor video data from processor 170, for example in the form of a sequenceof input image frames, and presents corresponding images to the user.The optics block 155 may include one or more optical elements, such asbut not exclusively a convex lens, a concave lens, a Fresnel lens, an LClens, a liquid lens, a pancake lens, an aperture, a grating, a filter, apolarizer and/or polarization converter, or any other suitable opticalelement. The optics block 155 may be configured to form a magnifiedimage of the light-emitting face 154 of the electronic display 153, oran area thereof, and may also correct for optical aberrations and otheroptical errors in the image light received from the electronic display153.

In some embodiments display 100 may use a varifocal lens which opticalpower may be dynamically adjusted to enhance user's experience. In someembodiments the varifocal lens may cooperate with the eye trackingsystem of HMD 100 to dynamically vary focusing properties of the HMDoptics to improve user's experience. In some embodiments a varifocallens may be constructed by stacking several PBP lenses of differingoptical powers, which in some embodiments may be interspersed withs-HWPs. By suitably selecting the optical powers of the PBP lenses, sucha multi-element or multi-layer LC PBP lens stack may be controlled toprovide a range of optical power with a step defined by thesmallest-power lens.

FIG. 7 schematically shows an example PBP lens system having opticalpower switchable between +D1 and −D1 diopters (D). In the illustratedembodiment it includes a circular polarizer (CPr) 220 followed by ans-HWP 230 followed by a PBP lens 210 of a nominal optical power D1. CPr220 may be for example RHC polarizer configured to convert inputpolarized or non-polarized light 201 to RHCP light 203. When s-HWP 230is in the ON state, it transmits the RHCP light 203 therethrough withouta polarization change, so that PBP lens 210 acts upon the RHCP light 205as a focusing lens with the optical power +D1, outputting convergingLHCP beam 207. When S-HWP 230 is in the OFF state, it changes the beampolarization to the orthogonal one, sending LHCP light to PBP lens 210.PBP lens 210 acts upon the LHCP light as a de-focusing lens with theoptical power (−D1), outputting diverging RHCP beam. Thus, an assemblyof a PBP lens in sequence with an s-HWP operates for CP light as aswitchable ±D1 lens. In some embodiments the PBP lens 210 may be active,and electrically switchable to a state with zero, or nearly zero,optical power, increasing the number of switchable optical power states.Stacking such two-element assemblies with different values of D1, forexample D1, D½, D¼, etc provides a multi-state varifocal lens.

FIG. 8 illustrates such a switchable LC stack that includes three PBPlenses 211, 212, and 213 with nominal optical powers of 1 D, 0.5 D, and0.25 D, as an example. The first PBP lens 211 is preceded by an RHCpolarizer 221 and a first s-HWP 231. A second s-HWP 232 is disposedbetween the first PBP lens 211 and the second PBP lens 212, and a thirds-HWP 233 is disposed between the second PBP lens 212 and the third PBPlens 213, which may be followed by a fourth s-HWP 234 and a clean-up RHCpolarizer 222. This arrangement is switchable between 8 focal distancescorresponding to −1.75 D, −1.25 D, −0.75 D, −0.25 D, 0.25 D, 0.75 D,1.25 D, and 1.75 D. In embodiments with active PBP lenses which opticalpower may be switched off, the optical power of the stack may be steppedwith a 0.25 D step from −1.75 D to +1.75 D. Other arrangements with adifferent, for example greater, number of PBP lenses may also be used.Increasing the number of PBP lenses enables broadening the adjustablefocal length range and/or decreasing the step in which the focal lengthmay be adjusted. In some embodiments, for example where some of the PBPlenses are active, the number of PBP lenses may exceed the number ofs-HWPs in the stack.

The varifocal lens stack 200, which is shown in FIG. 8 in an expandedview, includes three LC PBP lenses by way of example. Generally avarifocal lens stack of the type shown in FIG. 8 may include any numberN≥1 of PBP lenses, at least some of which preceded or followed by ans-HWP for switching. Elements of the stack may be sequentially laminatedon a common surface or carrier, with sequential elements optionallysharing a substrate, to provide an integrated LC stack, as illustratedin FIG. 9. A PBP lens preceded or followed by an s-HWP may be referredto as a switchable lens pair or a switchable lens unit.

FIG. 9 illustrates an example embodiment in which N≥2 switchable lenspairs 320 of a switchable LC stack 310 are sequentially laminated toform a varifocal LC block 350, which may be sandwiched between two CPpolarizers 321 and 322. In some embodiments the input CP polarizer 321may be commonly laminated to be a part of the varifocal LC block 350, ormay be a separate element, for example laminated upon a light-emittingface of an electronic display.

Referring to FIG. 10, we noticed that when a switchable LC stack of thetype illustrated in FIG. 8 or 9 is used in an HMD to provide dynamicallyadjustable focal distance, such as within the optical block 155 of HMD100, ring artifacts could sometimes be observed in the pupil plane oreyebox 157 of the HMD. These ring artifacts, which are schematicallyindicated in FIG. 10 as concentric rings 337, can be in the form ofpolychromatic ring-shaped modulations that are superimposed over, oradded to, an image being formed by the HMD. These ring artifacts may bedue to finite polarization selection efficiency of PBP lenses and/ors-HWPs, causing polarization leakage whereby undesired polarizationstate leaks into a subsequent element of the stack. Leaking of undesiredpolarization states to subsequent polarization-selective elements and/orthrough a clean-up or output polarizer may create a non-uniformtransmission pattern. We further discovered that the ring artifacts 337can be substantially eliminated or at least reduced when using aswitchable LC stack in which the first PBP lens receives unpolarizedlight

FIG. 11 illustrates an example stack 510 of polarization selectiveelements receiving unpolarized light 401. In the context of thisdescription, “unpolarized light” may refer to a light beam having adegree of polarization (DOP) of at most 30%, or preferably at most 10%.Stack 410 may be formed of a sequence of switchable lens pairs 320. Inoperation each of the PBP lenses of the stack, and each of theswitchable lens pairs 320 thereof, outputs both LHCP and RHCP lightbeams, adding an optical power D₁ to one of the output light beams andsubtracting the optical power D₁ from the other; here D₁ is the nominaloptical power of the i-th PBP lens of the stack being considered. Ifunpolarized light 401 is collimated, one of the CP output beams divergesand the other converges. Generally, stack 410 switchably convertsunpolarized image light 401 into two orthogonally polarized light beams403, 402 having differing convergence characteristics. The conversionperformed by the stack is switchable between a state where the stackadds optical power to the output RHCP beam while subtracting opticalpower from the output LHCP beam, and a state where the stack addsoptical power to the output RHCP light beam while subtracting opticalpower from the output RHCP light beam. In embodiments where theunpolarized light 401 is collimated, the stack outputs a convergentlight beam and a divergent light beam, as illustrated in FIG. 11 at 402and 403 respectively, each of which being switchable between twoorthogonal states of polarization. Thus stack 410 switchably convertsunpolarized light it receives into two orthogonally polarized lightbeams 402 and 403. Each of these two orthogonally polarized light beamsis switchable between two or more states of convergence, for example asdescribed above with reference to FIG. 8 for a stack with three PBPlenses of differing optical power. The final CP selection may be made bythe last s-HWP in the stack in cooperation with the output polarizer322. Consecutive polarization-selective elements of the stack, such asthe PBP lenses and the s-HWPs, may be in direct contact as shown in FIG.11, or they may be spaced apart from each other. In some embodiments,the output polarizer 322 may be a part of the stack, for example thelast element thereof in the direction of beam propagation. In someembodiments the output polarizer 322 may be external to the stack.

In some embodiments the number of PBP lenses in the stack 410 may differfrom the number of s-HWPs. Some embodiments of stack 410 may combine oneor more active, i.e. switchable, PBP lenses with switchable HWPs. Anactive LC PBP lens may be switched to a neutral state, in which it doesnot affect the optical power of the system and does not change thepolarization of light passing through the lens. In its active state, anactive LC PBP lens adds optical power to the system for CP light of onehandedness, and subtracts optical power from the system for CP light ofthe opposite handedness. PBP lenses that add optical power whenreceiving LHCP light may be referred to as LH PBP lenses, and PBP lensesthat add optical power when receiving RHCP light may be referred to asRH PBP lenses.

FIGS. 12A and 12B illustrate example switchable stacks including twoactive PBP lenses, 311 and 312, followed by an s-HWP 333 and a CPr 322in sequence. The first active PBP lenses 311 may have a nominal opticalpower D1, e.g. 1 D, while the second active PBP lenses 312 may have anominal optical power D2, e.g. 0.5 D. In FIG. 12A, the first and secondPBP lenses 311, 312 are of a same handedness, e.g. both being LH PBPlenses, and the stack includes a passive HWP disposed between the twoPBP lenses for converting circular polarizations passed through thefirst PBP lens 311 back to its input handedness prior to entering thesecond PBP lens 312. A passive HWP between two active PBP lenses may notbe needed however if consecutive PBP lenses are of opposite handedness,i.e. where an RH PBP lens is immediately followed or preceded by an LHPBP lens. In FIG. 12B, the second active PBP lens 312 has the oppositehandedness of the first PBP lens 311, e.g. it may be a RH PBP lens ifthe first PBP lens 311 is a LH PBP lens; accordingly, the passive HWP331 between the two PBP lenses 311, 312 is omitted in FIG. 12B.

When both active PBP lenses 311, 312 are in the neutral state, each ofthe stacks shown in FIGS. 12A and 12B acts substantially as a CPpolarizer for input unpolarized light 301, with a zero total opticalpower. When both active PBP lenses 311, 312 are in their active state,the output s-HWP 333 receives two CP beams 302, 303 that have oppositehandedness and different convergence/divergence beam characteristics,one with an added positive optical power of (D1+D2), e.g. 1.5 D, andanother with an added negative optical power of −(D1+D2), e.g. −1.5 D.By switching the output s-HWP 333 on and off, the total optical power ofthe stack is switched between +(D1+D2) and −(D1+D2). When one of theactive PBP lenses 311 and 312 is in its active state while the other isin its passive state, the total optical power of the stack may beswitched by the output s-HWP 333 between +\−D2 if the first active PBPlens 311 is in its neutral state, e.g. +\−0.5 D, or between +/−D1 if thesecond active PBP lens 312 is in its neutral state, e.g. between +\−1 D.Thus, each of the stacks shown in FIGS. 12A and 12B may operate as avarifocal lens with its optical power switchable between 7 differentsates, (−D1+D2), −D1, −D2, 0, +D2, +D1, and +(D1+D2), when D1 D2. Thenumber of states with different optical powers that the stacks may beswitched between may be increased by adding PBP lenses to the stack and,if the PBP lenses are of the same handedness, with additional HWPsfollowing or preceding them.

Referring to FIG. 13, there is illustrated an example wearable displaysystem or HMD 400 utilizing a switchable LC stack 510. The switchable LCstack 510 may be embodied, for example, as described above withreference to FIGS. 11-12B, to be operable as a varifocal lens, and maylack an input polarizer so as to receive unpolarized light. In FIG. 12,elements functionally same or similar to those described above withreference to FIGS. 6 to 12B are indicated with the same referencenumerals. HMD 400 may be viewed as an embodiment or modification of HMD100, in which an electronic display 453 emits unpolarized light, and theoptics block includes a pancake lens 455 disposed downstream of theswitchable LC stack 510. HMD 400 may include other optical elements inplace or additionally to the pancake lens 455 as described above withreference to optical block 155. It was observed that removal of an inputpolarizer upstream of a first PBP lens of the stack 510, so that theswitchable LC stack 510, or at least the first PBP element thereof,receives unpolarized image light from display 453, substantiallyeliminates the appearance of ring artifacts at the eyebox.

Referring to FIG. 14, there is illustrated a wearable display system orHMD 500 having an electronic display 553 that emits image light that islinearly polarized. An example is an LC-based electronic display inwhich light is modulated substantially by controlling its polarization,and which therefore typically outputs polarized light. In suchembodiments, a depolarizer 530 may be provided upstream of theswitchable LC stack 510 to de-polarize the polarized light from thedisplay. In some embodiments where the polarization of the image lightemitted by the display is linear, depolarizer 530 may be configuredspecifically to depolarize linearly polarized light. In embodimentswhere the polarization of the image light emitted by the display iscircular, depolarizer 530 may be configured specifically to depolarizeCP light. It was observed that the addition of the depolarizer 530 inthe optical path between display 553 and the first PBP lens of theswitchable LC stack 510, so that the switchable LC stack 510 receivesunpolarized image light, substantially eliminates or at least lessensthe ring artifacts. In FIG. 14, elements functionally similar to thosedescribed above with reference to FIGS. 6 to 12 are indicated with thesame reference numerals. It will be appreciated that the HMDarchitecture shown in FIG. 13 allows for various modifications. Forexample, in some embodiments the optics block of the HMD may includeother optical elements in addition to those shown in FIG. 14. In someembodiments the shown optical elements or devices may be disposed in adifferent order. FIG. 15 illustrates an example embodiment 600 of HMD500 in which the depolarizer 530 and the switchable LC stack 510 aredisposed downstream of the pancake lens 455.

In some embodiments, the depolarizer 530 may be included in a switchableLC stack, such as stack 510 or 410 described above, and may form anintegrated block 610 therewith as illustrated in FIG. 16. In someembodiments, a depolarizer may be a separate element. In someembodiments a depolarizer may be laminated on another component of theHMD, for example upon the pixelated face of display 553 that emitspolarized image light.

In embodiments where depolarizer 530 operates as a diffusor thatspatially perturbs the optical phase of light passing therethrough in arandom or quasi-random fashion, it may be preferable to position thedepolarizer proximate to display 553. In some embodiments thedepolarizer may be positioned at a distance d from the display whereeach location receives image light emitted by at most only a few, forexample four, neighboring pixels, e.g. pixels immediately adjacent toeach other. In some embodiments the depolarizer-display distanced≤p·cot(β), where p is the pixel pitch and β is one half of an emittingangle of a pixel. In some embodiments the depolarizer may be placed in adirect contact with the pixelated face of display 553, e.g. a liquidcrystal display panel or an LED array panel. In some embodiments thedepolarizer may be laminated onto the display 553. In some embodimentslayers of the switchable LC stack 510 may be laminated upon display 553over the depolarizer.

FIG. 17 schematically illustrates an example depolarizer 700 that may beused as depolarizer 530 in some embodiments of the present disclosure.Depolarizer 700 may be in the form of, or include, a waveplate having abirefringent layer of some retardance Q in which the in-planeorientation of its optic axis 701, as described for example by itsazimuth angle in the plane of the layer, spatially varies across thewaveplate. In some embodiments the azimuth angle of the optic axis 701changes randomly, pseudo-randomly, or in accordance with some suitablefunction or rule, by up to +\−90° and more across an area of thewaveplate that is several times smaller than the total operating area ofthe waveplate. In some embodiment a length l over which the optic axis701 may rotate by at least 90 degrees may be at least 2 times smaller,and preferably 5-10 times smaller than the size a of an operating areaof the waveplate in at least one direction across the waveplate, e.g.either in x-axis or the y-axis directions illustrated in the figure. Insome embodiment the length l, which may be referred to as thecorrelation length, may be at least 2 times smaller, and preferably 5-10times smaller than the size of the operating area of the waveplate inany direction across the waveplate. Here the operating area of awaveplate is an area that is illuminated by a beam of light on which thewaveplate operates. In some embodiments, for example when thedepolarizer 700 is placed adjacent or close to an electronic display,such as electronic display 553 of the display system 500, thecorrelation length 1 of the optic axis of depolarizer 700 may be smallerthan a pixel size b of the display, for example 2-5 times smaller, sothat light from each pixel of the display assumes a range ofpolarization states and gets depolarized.

Depolarizer 700 may be formed with a layer of birefringent material ofuniform in-plane birefringence and same retardance Q across the layer.In some embodiments depolarizer 700 may be configured for depolarizinglinearly polarized light. In some embodiments the retardance Qcorresponds to a half-wave retardance, so that depolarizer 700 is a HWPwith a spatially varying in-plane optic axis orientation. In someembodiments the in-plane spatial variation of the optic axis orientationmay be random or pseudorandom, with an in-plane correlation length 1 atleast 5 times smaller than a characteristic size a of the operating areaof the depolarizer, i.e. its size in each x- and y-in-plane dimensionsor an expected diameter of the incident light beam. In some embodimentsthe in-plane correlation length 1 may be at least 2 times smaller than apixel size b of an electronic display to which the depolarizer iscoupled. In some embodiments the in-plane correlation length 1 may be atleast 5 times smaller than a pixel size b of an electronic display towhich the depolarizer is coupled. Embodiments in which the orientationof the optic axis 701 varies in somewhat regular manner across thewaveplate may also be envisioned, although may result in the appearanceof visual artifacts in some circumstances.

In some embodiment depolarizer 700 may be in the form of an LCwaveplate, such as an LC HWP, with the in-plane orientation of its opticaxis 701, as may be defined by the LC director's orientation, varyingspatially in the plane of the LC layer. In some embodiments thisvariation may be random or pseudorandom in either in-plane direction ofthe LC layer, with a size c of an area segment 703 playing the role ofthe optic axis correlation length 1. In some embodiments the LCwaveplate may be comprised of a plurality of area segments 703 whichhave substantially the same retardance, e.g. that of a HWP, butdiffering in-plane orientation of the optic axis or LC director 701. Insome embodiments the in-plane orientation of the optic axis variesrandomly or pseudo-randomly from one area segment 703 to the next. Inthe context of this description the term “pseudorandom” refers to astatistical process that may be emulated using a computer-implementedpseudo-random number generator. In some embodiments the LC layercomprises nematic LC material. In some embodiments the nematic LCmaterial may be twisted nematic LC for broad-spectrum operation. In someembodiment the LC layer may be stabilized by a polymer network.

In some embodiments the following “phase-randomizing” condition may besatisfied

|Σ₁ ^(N) {right arrow over (e)} _(i)|≤1/N,

where N is the number of area segments across the operating area of thewaveplate in either in-plane dimension (i.e. along the x-axis or they-axis, or along any diameter of a circular waveplate), and {right arrowover (e)}_(i) is an in-plane vector of unit length which directioncorresponds to a characteristic in-plane direction of the optic axis ofan i-th area segment 701. In some embodiment the size c of an areasegment 703 may be smaller than the pixel pitch p of an electronicdisplay to which the depolarizer is coupled when placed adjacent to theelectronic display. In some embodiment the size c of an area segment 703may be at least 2 times smaller than the pixel pitch p. In someembodiment the size c of an area segment 703 may be at least 5 timessmaller than the pixel pitch p.

Referring to FIG. 18, the LC depolarizer 700 may be fabricated byoptionally depositing an alignment layer 800 of a photo-alignablematerial on a substrate, such as substrate 30 illustrated in FIG. 1, andexposing the alignment layer 800 to UV light having spatially varyingpolarization in order to align molecules of the alignment layer in aspatial varying direction as described above. In some embodiments thepolarized UV beam may be focused upon an area segment 803 of thealignment layer 800 so as to align the material of the alignment layerin an alignment direction 801 defined by the polarization of the UVbeam. The UV beam may then be stepped from segment to segment across thealignment layer, changing its polarization for each step in a random orpseudo-random fashion, so as to form an array of area segments 803 ofthe alignment layer 800 having a random or pseudorandom orientation ofthe alignment direction 801. In some embodiments, a mask may be steppedand used to pseudo-randomly vary the linear polarization of the UV beamand/or the exposure area while the beam polarization in varied, or moregenerally a polymerizing beam at a polymerizing wavelength. Next, alayer of LC material may be deposited, for example spun-coated, upon thealignment layer 800, with the LC director aligning in each area segment803 along the alignment direction 801 in that segment. In someembodiments the LC material may then be polymerized, for example inpresence of nitrogen under UV light, to fix the LC direction orientationin the random or pseudo-random pattern. In some embodiments a secondsubstrate may be optionally affixed to the first substrate to cover theLC layer. In some embodiments the LC material comprises nematic LC. Insome embodiments the LC material comprises chiral or twisted nematic LC.In some embodiments the pitch of the chiral LC material of the LC layer,i.e. the distance it takes for the LC director to rotate one full turnin the helix in the direction normal to the LC layer, may be selectedfor providing an HWP retardance for visible light. In some embodimentschiral dopants or chiral materials may be added to the LC material.

Referring to FIGS. 19, 20A, and 20B, in some embodiments a retardationcompensation method may be used to provide the HWP operation in a broadwavelength range, e.g. across the RGB spectrum. In accordance with anembodiment of the method, a broad-band depolarizer 900 may comprise adepolarizing bilayer 920 disposed over an optional substrate 910. Thedepolarizing bilayer 920 may be flat or curved, and may be formed of twodepolarizing layers 921, 922 of chiral liquid crystal stacked on top ofeach other. The LC director of each layer is indicated at 903 in FIGS.20A and 20B in projection on the plane of the layers. The first layer921 has the LC director 903 (FIGS. 20A, 20B) with a first sense of twistin a thickness direction, wherein the thickness direction is normal tothe substrate 910 (the z-axis direction in FIG. 19). The second layer922 of the twisted LC material is disposed over the first layer 921,with the LC director 903 having a second sense of twist in the thicknessdirection, wherein the second sense of twist in the second layer isopposite to the first sense of twist in the first layer. The lower layer921 may be disposed over an alignment layer 915 and an optionalsubstrate 910 that may be optically transparent. The LC director 903 ofthe first layer at a particular distance from the substrate 910, forexample at an interface with the second layer 922 as schematicallyillustrated with a solid arrow in FIG. 20A, has a direction that variesrandomly or pseudo-randomly in the plane of the substrate (x,y). Thealignment layer 915 may be randomly photoaligned, for example asdescribed above with reference to FIG. 18. The orientation of the LCdirector 903 in a vertical (x,z) cross-section of an n-th segment ofdepolarizer 900 (FIG. 19, the LC director not shown) may be described byan azimuth angle ϕ(n,x,z) that may vary according to equation (2):

$\begin{matrix}{{\varphi \left( {n,x,z} \right)} = \left\{ \begin{matrix}{\Phi_{n} + {\Phi \; z\text{/}d}} & {{{{if}\mspace{14mu} 0} \leq z \leq d}\mspace{14mu}} \\{\Phi_{n} + {\Phi \left( {2 - \frac{z}{d}} \right)}} & {{{if}\mspace{14mu} d} < z \leq {2d}}\end{matrix} \right.} & (2)\end{matrix}$

Here d is the thickness of each depolarizing layer 921, 922, T is anazimuth angle of the LC director in the first depolarizing layer 921 atthe border with the alignment layer 915 in the n-th area segment ofdepolarizer 900, and may be referred to as a base angle of the LCdirector of n-th segment. The base angle values {Φ_(n)} may form arandom or pseudo-random set with a uniform probability distributionacross a 180 degrees range.

FIGS. 20A and 20B schematically illustrate, in a plan view, theorientation of the LC director 903 in an n-th segment at differentdistances from the alignment layer 915. In FIG. 20A, the orientation ofthe LC director 903 is schematically shown with a dotted arrow at theborder with the alignment layer 915, with a dashed arrow in the middleof the first layer 921, and with a solid arrow at the border with thesecond depolarizing layer 922. In FIG. 20B, the orientation of the LCdirector 903 is schematically shown with a dotted arrow at the borderwith the first depolarizing layer 921, with a dashed arrow in the middleof the second depolarizing layer 922, and with a solid arrow at the topsurface of the second depolarizing layer 922. Thus, in this example theLC director 903 in the first layer 921 and the second layer 922 haveopposite twist sense and equal absolute value of the twist angle 1 ineach layer, with the total twist angle across two layers of zero. Thefirst and second layers 921, 922 may each have the thickness of ahalf-wave plate in the wavelength range of operation, providing a totalretardation of a HWP for the bilayer 920.

The opposite twist sense along the layer thickness may be obtained byadding chiral dopants of opposite chirality to a curable polymericliquid crystal mixture (CPLCM). Chiral dopants that have right-handed(RH) chirality may be used to form one layer, and chiral dopants thathave left-handed (LH) chirality may be used to form the other layer. Theprocess may include depositing a linear photopolymerizable polymer (LPP)onto a suitable substrate to form an alignment layer, recording adesired random or pseudo-random alignment pattern into the alignmentlayer, coating the alignment layer with a first CPLCM layer doped withsuitable chiral molecules of a first chirality, for example RH, to formthe first depolarizing layer 921, and depositing a second CPLCM layerdoped with suitable chiral molecules of a second chirality, for exampleLH, over the first CPLCM layer to form the second depolarizing layer922. In other embodiments dopants of LH chirality may be used for thefirst layer, and dopants of RH chirality for the second layer. Examplesof suitable chiral dopants include chiral dopants R811 (right hand) and5811 (left hand) that are available from Merck & CO., Inc. The degree oftwist, or twist angle 1 in each layer, is determined by the layerthickness d and the pitch of the chiral LC solution, i.e. the distanceit takes for the LC director to rotate one full turn in the helix in thedirection normal to the LC layer. The chiral pitch (p) is related to theconcentration (c) and helical twisting power (HTP) of the chiral dopant(p=1/(HTP*c).

It will be appreciated that other types of depolarizers may be used inNEDs and other compact display systems in conjunction with switchable LCstacks, including but not limited to LC based depolarizers, non-LCdepolarizers, and depolarizers capable of depolarizing CP light.Furthermore many variations and modifications of example embodimentsdescribed above will be apparent to those of ordinary skill in the artfrom the foregoing description and accompanying drawings. For example,in some embodiments the switchable multi-layer LC stack operating withunpolarized incident light may include other PBP optical elements, suchas PBP gratings (see e.g. FIG. 3B) in addition to, or instead of, thePBP lenses described above. For example, in some embodiments each PBPlens in the switchable stack 410 or 510 may be replaced with apolarization grating, such as a PBP grating, to provide a switchableimage light shifter or deflector. Similarly, in some embodiments eachactive PBP lens in the example switchable stacks illustrated in FIGS.12A, 12B may be replaced with an active polarization grating, such as anactive LC PBP grating. In some embodiments using such switchable beamdeflectors with unpolarized light may reduce visual artifacts, eithercircular or linear, which may occur due to polarization leaking. In someembodiments a de-polarizer may be used as a part of an HDM in front of anon-switchable LC stack that may include a polarizer or a polarizationselector.

Embodiments of the present disclosure may include, or be implemented inconjunction with, an artificial reality system. An artificial realitysystem adjusts sensory information about outside world obtained throughthe senses such as visual information, audio, touch (somatosensation)information, acceleration, balance, etc., in some manner beforepresentation to a user. By way of non-limiting examples, artificialreality may include virtual reality (VR), augmented reality (AR), mixedreality (MR), hybrid reality, or some combination and/or derivativesthereof. Artificial reality content may include entirely generatedcontent or generated content combined with captured (e.g., real-world)content. The artificial reality content may include video, audio,somatic or haptic feedback, or some combination thereof. Any of thiscontent may be presented in a single channel or in multiple channels,such as in a stereo video that produces a three-dimensional effect tothe viewer. Furthermore, in some embodiments, artificial reality mayalso be associated with applications, products, accessories, services,or some combination thereof, that are used to, for example, createcontent in artificial reality and/or are otherwise used in (e.g.,perform activities in) artificial reality. The artificial reality systemthat provides the artificial reality content may be implemented onvarious platforms, including a wearable display such as an HMD connectedto a host computer system, a standalone HMD, a near-eye display having aform factor of eyeglasses, a mobile device or computing system, or anyother hardware platform capable of providing artificial reality contentto one or more viewers.

Referring to FIG. 21A, an HMD 1100 is an example of an AR/VR wearabledisplay system which encloses the user's face, for a greater degree ofimmersion into the AR/VR environment. The HMD 1100 may be an embodimentof the wearable display system 400 of FIG. 13 or the wearable displaysystem 500 of FIG. 14, for example. The function of the HMD 1100 is toaugment views of a physical, real-world environment withcomputer-generated imagery, and/or to generate the entirely virtual 3Dimagery. The HMD 1100 may include a front body 1102 and a band 1104. Thefront body 1102 is configured for placement in front of eyes of a userin a reliable and comfortable manner, and the band 1104 may be stretchedto secure the front body 1102 on the user's head. A display system 1180may be disposed in the front body 1102 for presenting AR/VR imagery tothe user. Sides 1106 of the front body 1102 may be opaque ortransparent.

In some embodiments, the front body 1102 includes locators 1108 and aninertial measurement unit (IMU) 1110 for tracking acceleration of theHMD 1100, and position sensors 1112 for tracking position of the HMD1100. The IMU 1110 is an electronic device that generates dataindicating a position of the HMD 1100 based on measurement signalsreceived from one or more of position sensors 1112, which generate oneor more measurement signals in response to motion of the HMD 1100.Examples of position sensors 1112 include: one or more accelerometers,one or more gyroscopes, one or more magnetometers, another suitable typeof sensor that detects motion, a type of sensor used for errorcorrection of the IMU 1110, or some combination thereof. The positionsensors 1112 may be located external to the IMU 1110, internal to theIMU 1110, or some combination thereof.

The locators 1108 are traced by an external imaging device of a virtualreality system, such that the virtual reality system can track thelocation and orientation of the entire HMD 1100. Information generatedby the IMU 1110 and the position sensors 1112 may be compared with theposition and orientation obtained by tracking the locators 1108, forimproved tracking accuracy of position and orientation of the HMD 1100.Accurate position and orientation may help presenting appropriatevirtual scenery to the user as the latter moves and turns in 3D space.

The HMD 1100 may further include a depth camera assembly (DCA) 1111,which captures data describing depth information of a local areasurrounding some or all of the HMD 1100. To that end, the DCA 1111 mayinclude a laser radar (LIDAR), or a similar device. The depthinformation may be compared with the information from the IMU 1110, forbetter accuracy of determination of position and orientation of the HMD1100 in 3D space.

The HMD 1100 may further include an eye tracking system 1114 fordetermining orientation and position of user's eyes in real time. Theobtained position and orientation of the eyes may allow the HMD 1100 todetermine the gaze direction of the user and to adjust the imagegenerated by the display system 1180 accordingly. In one embodiment, thevergence, that is, the convergence angle of the user's eyes gaze, isdetermined. The determined gaze direction and vergence angle may also beused for real-time compensation of visual artifacts dependent on theangle of view and eye position. Furthermore, the determined vergence andgaze angles may be used for interaction with the user, highlightingobjects, bringing objects to the foreground, creating additional objectsor pointers, etc. An audio system may also be provided including e.g. aset of small speakers built into the front body 1102.

Referring to FIG. 21B, an AR/VR system 1150 may be an exampleimplementation of the wearable display system 400 of FIG. 13, thewearable display system 500 of FIG. 14, or the wearable display system600 of FIG. 15. The AR/VR system 1150 includes the HMD 1100 of FIG. 21A,an external console 1190 storing various AR/VR applications, setup andcalibration procedures, 3D videos, etc., and an input/output (I/O)interface 1115 for operating the console 1190 and/or interacting withthe AR/VR environment. The HMD 1100 may be “tethered” to the console1190 with a physical cable, or connected to the console 1190 via awireless communication link such as Bluetooth®, Wi-Fi, etc. There may bemultiple HMDs 1100, each having an associated I/O interface 1115, witheach HMD 1100 and I/O interface(s) 1115 communicating with the console1190. In alternative configurations, different and/or additionalcomponents may be included in the AR/VR system 1150. Additionally,functionality described in conjunction with one or more of thecomponents shown in FIGS. 21A and 21B may be distributed among thecomponents in a different manner than described in conjunction withFIGS. 21A and 21B in some embodiments. For example, some or all of thefunctionality of the console 1190 may be provided by the HMD 1100, andvice versa. The HMD 1100 may be provided with a processing modulecapable of achieving such functionality.

As described above with reference to FIG. 21A, the HMD 1100 may includethe eye tracking system 1114 (FIG. 21B) for tracking eye position andorientation, determining gaze angle and convergence angle, etc., the IMU1110 for determining position and orientation of the HMD 1100 in 3Dspace, the DCA 1111 for capturing the outside environment, the positionsensor 1112 for independently determining the position of the HMD 1100,and the display system 1180 for displaying AR/VR content to the user.The display system 1180 includes (FIG. 21B) an electronic display 1125,for example and without limitation, a liquid crystal display (LCD), anorganic light emitting display (OLED), an inorganic light emittingdisplay (ILED), an active-matrix organic light-emitting diode (AMOLED)display, a transparent organic light emitting diode (TOLED) display, aprojector, or a combination thereof. The display system 1180 furtherincludes an optics block 1130, whose function is to convey the imagesgenerated by the electronic display 1125 to the user's eye. The opticsblock may include various lenses, e.g. a refractive lens, a Fresnellens, a diffractive lens, an active or passive Pancharatnam-Berry phase(PBP) lens, a liquid lens, a liquid crystal lens, etc., apupil-replicating waveguide, grating structures, coatings, etc. Thedisplay system 1180 may further include a varifocal module 1135, whichmay be a part of the optics block 1130. The function of the varifocalmodule 1135 is to adjust the focus of the optics block 1130 e.g. tocompensate for vergence-accommodation conflict, to correct for visiondefects of a particular user, to offset aberrations of the optics block1130, etc. The varifocal module 1135 may be for example embodied with aswitchable PBP lens stack such as those illustrated in FIGS. 11, 12A,12B, 15, their variations and combinations, and may include adepolarizer in some embodiments.

The I/O interface 1115 is a device that allows a user to send actionrequests and receive responses from the console 1190. An action requestis a request to perform a particular action. For example, an actionrequest may be an instruction to start or end capture of image or videodata or an instruction to perform a particular action within anapplication. The I/O interface 1115 may include one or more inputdevices, such as a keyboard, a mouse, a game controller, or any othersuitable device for receiving action requests and communicating theaction requests to the console 1190. An action request received by theI/O interface 1115 is communicated to the console 1190, which performsan action corresponding to the action request. In some embodiments, theI/O interface 1115 includes an IMU that captures calibration dataindicating an estimated position of the I/O interface 1115 relative toan initial position of the I/O interface 1115. In some embodiments, theI/O interface 1115 may provide haptic feedback to the user in accordancewith instructions received from the console 1190. For example, hapticfeedback can be provided when an action request is received, or theconsole 1190 communicates instructions to the I/O interface 1115 causingthe I/O interface 1115 to generate haptic feedback when the console 1190performs an action.

The console 1190 may provide content to the HMD 1100 for processing inaccordance with information received from one or more of: the IMU 1110,the DCA 1111, the eye tracking system 1114, and the I/O interface 1115.In the example shown in FIG. 21B, the console 1190 includes anapplication store 1155, a tracking module 1160, and a processing module1165. Some embodiments of the console 1190 may have different modules orcomponents than those described in conjunction with FIG. 21B. Similarly,the functions further described below may be distributed amongcomponents of the console 1190 in a different manner than described inconjunction with FIGS. 21A and 21B.

The application store 1155 may store one or more applications forexecution by the console 1190. An application is a group of instructionsthat, when executed by a processor, generates content for presentationto the user. Content generated by an application may be in response toinputs received from the user via movement of the HMD 1100 or the I/Ointerface 1115. Examples of applications include: gaming applications,presentation and conferencing applications, video playback applications,or other suitable applications.

The tracking module 1160 may calibrate the AR/VR system 1150 using oneor more calibration parameters and may adjust one or more calibrationparameters to reduce error in determination of the position of the HMD1100 or the I/O interface 1115. Calibration performed by the trackingmodule 1160 also accounts for information received from the IMU 1110 inthe HMD 1100 and/or an IMU included in the I/O interface 1115, if any.Additionally, if tracking of the HMD 1100 is lost, the tracking module1160 may re-calibrate some or all of the AR/VR system 1150.

The tracking module 1160 may track movements of the HMD 1100 or of theI/O interface 1115, the IMU 1110, or some combination thereof. Forexample, the tracking module 1160 may determine a position of areference point of the HMD 1100 in a mapping of a local area based oninformation from the HMD 1100. The tracking module 1160 may alsodetermine positions of the reference point of the HMD 1100 or areference point of the I/O interface 1115 using data indicating aposition of the HMD 1100 from the IMU 1110 or using data indicating aposition of the I/O interface 1115 from an IMU included in the I/Ointerface 1115, respectively. Furthermore, in some embodiments, thetracking module 1160 may use portions of data indicating a position orthe HMD 1100 from the IMU 1110 as well as representations of the localarea from the DCA 1111 to predict a future location of the HMD 1100. Thetracking module 1160 provides the estimated or predicted future positionof the HMD 1100 or the I/O interface 1115 to the processing module 1165.

The processing module 1165 may generate a 3D mapping of the areasurrounding some or all of the HMD 1100 (“local area”) based oninformation received from the HMD 1100. In some embodiments, theprocessing module 1165 determines depth information for the 3D mappingof the local area based on information received from the DCA 1111 thatis relevant for techniques used in computing depth. In variousembodiments, the processing module 1165 may use the depth information toupdate a model of the local area and generate content based in part onthe updated model.

The processing module 1165 executes applications within the AR/VR system1150 and receives position information, acceleration information,velocity information, predicted future positions, or some combinationthereof, of the HMD 1100 from the tracking module 1160. Based on thereceived information, the processing module 1165 determines content toprovide to the HMD 1100 for presentation to the user. For example, ifthe received information indicates that the user has looked to the left,the processing module 1165 generates content for the HMD 1100 thatmirrors the user's movement in a virtual environment or in anenvironment augmenting the local area with additional content.Additionally, the processing module 1165 performs an action within anapplication executing on the console 1190 in response to an actionrequest received from the I/O interface 1115 and provides feedback tothe user that the action was performed. The provided feedback may bevisual or audible feedback via the HMD 1100 or haptic feedback via theI/O interface 1115.

In some embodiments, based on the eye tracking information (e.g.,orientation of the user's eyes) received from the eye tracking system1114, the processing module 1165 determines resolution of the contentprovided to the HMD 1100 for presentation to the user on the electronicdisplay 1125. The processing module 1165 may provide the content to theHMD 1100 having a maximum pixel resolution on the electronic display1125 in a foveal region of the user's gaze. The processing module 1165may provide a lower pixel resolution in other regions of the electronicdisplay 1125, thus lessening power consumption of the AR/VR system 1150and saving computing resources of the console 1190 without compromisinga visual experience of the user. In some embodiments, the processingmodule 1165 can further use the eye tracking information to adjust whereobjects are displayed on the electronic display 1125 to preventvergence-accommodation conflict and/or to offset optical distortions andaberrations.

The hardware used to implement the various illustrative logics, logicalblocks, modules, and circuits described in connection with the aspectsdisclosed herein may be implemented or performed with a general purposeprocessor, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA) orother programmable logic device, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described herein. A general-purpose processor maybe a microprocessor, but, in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine. Aprocessor may also be implemented as a combination of computing devices,e.g., a combination of a DSP and a microprocessor, a plurality ofmicroprocessors, one or more microprocessors in conjunction with a DSPcore, or any other such configuration. Alternatively, some steps ormethods may be performed by circuitry that is specific to a givenfunction.

The present disclosure is not to be limited in scope by the specificembodiments described herein, and various other embodiments andmodifications will become evident to the skilled reader from the presentdisclosure. Thus, such other embodiments and modifications are intendedto fall within the scope of the present disclosure. Further, althoughthe present disclosure has been described herein in the context of aparticular implementation in a particular environment for a particularpurpose, those of ordinary skill in the art will recognize that itsusefulness is not limited thereto and that the present disclosure may bebeneficially implemented in any number of environments for any number ofpurposes. For example, embodiments described herein with reference towearable display systems such as HMDs may also be implemented in otherdisplay systems, such as but not exclusively heads-up displays (HUDs)and heads-down displays. Accordingly, the claims set forth below shouldbe construed in view of the full breadth and spirit of the presentdisclosure as described herein.

What is claimed is:
 1. A display system comprising: a source ofunpolarized image light; a stack of polarization-selective opticalelements disposed to receive the unpolarized image light and operable toswitchably convert the unpolarized image light into two orthogonallypolarized light beams; and, an output polarizer disposed to receive thetwo orthogonally polarized light beams and configured to select one ofthe two orthogonally polarized light beams for forming an image.
 2. Thedisplay system of claim 1 configured to be mounted upon a user's headfor near-eye display of images, wherein the stack is absent an inputpolarizer.
 3. The display system of claim 1, wherein the stack comprisesone or more Pancharatnam-Berry phase (PBP) optical elements, and whereinat least one of the one or more PBP optical elements is disposed toreceive the unpolarized image light.
 4. The display system of claim 3wherein the stack comprises one or more switchable half-wave plates(HWP).
 5. The display system of claim 4, wherein the one or more PBPoptical elements comprise a plurality of PBP lenses of differing nominaloptical power, and wherein the stack is configured to operate as avary-focal lens.
 6. The display system of claim 4 wherein the pluralityof PBP lenses comprises a switchable liquid crystal (LC) PBP lens. 7.The display system of claim 3 wherein the plurality of PBP lensescomprises one or more passive PBP lenses, each followed by a switchableHWP.
 8. The display system of claim 4 wherein the one or more PBPoptical elements comprise a polarization grating.
 9. The display systemof claim 8 wherein the polarization grating is directly followed by aswitchable HWP.
 10. The display system of claim 8 wherein thepolarization grating comprises an LC PBP grating switchable to anon-diffracting state.
 11. The display system of claim 1 wherein thesource of unpolarized image light comprises an electronic displayconfigured to emit unpolarized light.
 12. The display system of claim 1wherein the source of unpolarized image light comprises an electronicdisplay configured to emit polarized image light, and a depolarizerdisposed in an optical path between the electronic display and thestack.
 13. The display system of claim 12 wherein the polarized imagelight is linearly polarized, and wherein the depolarizer comprises ahalf-wave plate (HWP) with a spatially randomized in-plane optic axis.14. The display system of claim 13 wherein the depolarizer is disposedadjacent to the electronic display.
 15. The display system of claim 12wherein the stack comprises a sequence of LC PBP optical elements, eachpaired with a switchable LC HWP.
 16. The display system of claim 12wherein at least one of the polarization-selective elements of the stackcomprises a switchable LC PBP optical element.
 17. A method forpolarization-based processing of image light in a display system, themethod comprising: passing unpolarized image light through a sequence ofpolarization-selective optical elements to obtain two orthogonallypolarized light beams, each of which being switchable in at least onebeam characteristic; and, using an optical polarizer disposed downstreamof the sequence of polarization-selective optical elements to select oneof the two orthogonally polarized light beams as an output light beamfor forming an image.
 18. The method of claim 17 comprising passingpolarized image light through a depolarizer to obtain the unpolarizedimage light for providing to the sequence of polarization-selectiveoptical elements.
 19. The method of claim 17 wherein the passingcomprises passing the unpolarized image light through a sequence of PBPlenses of different nominal optical powers.
 20. The method of claim 18wherein the passing comprises passing the unpolarized image lightthrough a sequence of polarization gratings.